What is the problem of words in the context of formal languages?

The problem of words is related to the equivalence of words in a formal language.
The problem of words is related to the construction of a formal language.
The problem of words is related to the translation of words from one formal language to another.
a) Only statement 1 is correct.
b) Only statement 2 is correct.
c) Only statement 3 is correct.
d) Statements 1 and 2 are correct.
e) Statements 1 and 3 are correct.